Image sensor with improved color crosstalk

ABSTRACT

An image sensor comprises a substrate of a first conductivity type. First and second pixels are arrayed over the substrate. A potential barrier is formed in a region of the substrate corresponding to the first pixel but not in a region of the substrate corresponding to the second pixel. The second pixel is responsive to a color having a wavelength longer than the color to which the first pixel is responsive. The potential barrier is doped with dopants by a high energy ion implantation dopants or by an ion implantation or diffusion during epitaxial growth of the P-type epitaxial layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present invention claims priority of Korean patent applicationnumber 10-2006-0099759, filed on Oct. 13, 2006, which is incorporated byreference in its entirety.

BACKGROUND OF THE INVENTION

The present invention relates to a solid-state image sensor, moreparticularly, to a complementary metal-oxide semiconductor (CMOS) imagesensor including small pixels covered with light absorbing colorfilters.

A typical pixel of a modern CMOS image sensor includes a photodiode,more specifically, a pinned photodiode, and four transistors. Thephotodiode collects photo-generated charge that is later transferredonto a floating diffusion (FD) node at a suitable moment by a chargetransfer transistor. The FD node functions as a charge detection node.Prior to the charge transfer, the FD node needs to be reset to asuitable reference voltage. The reset causes kTC noise, which would benormally added to a signal appearing on the FD node. Thus, it isnecessary to read the voltage on the FD node twice, the first timebefore the charge transfer, and the second time after the chargetransfer. This operation is called CDS (Correlated Double Sampling). TheCDS operation allows sensing of only the voltage difference on the nodecaused by the transferred charge from the photodiode.

A source follower (SF) transistor senses the voltage on the FD nodethrough a gate of the SF transistor connected to the FD node, a drainthereof connected to a power voltage (Vdd) terminal, and a sourcethereof connected to a common column sense line via addressingtransistor. For this reason, incorporating 4 transistors in each pixelof a standard CMOS image sensor is generally necessary. U.S. Pat. No.5,625,210 issued to Paul P. Lee et al. in the name of “Active PixelSensor Integrated with Pinned Photodiode” describes one exemplary 4Tpixel circuit with a pinned photodiode.

In modern CMOS sensor designs, the circuitry for several photodiodes maybe shared as can be found exemplarily in U.S. Pat. No. 6,657,665 B1,issued to R. M. Guidash et al., entitled “Active Pixel Sensor with WiredFloating Diffusions and Shared Amplifier.” In this patent application, adual pixel includes two photodiodes located in adjacent rows of a sensorimage array and sharing the same circuitry.

The color sensing in most modern CMOS image sensors is accomplished byplacing suitable color filters over the photodiodes as is shown inFIG. 1. A blue color filter 101 absorbs green and red light and letsonly the blue light photons to enter the photodiode area below.Similarly, a green color filter 102 absorbs blue and red light and letsonly the green light photons to enter the silicon bulk below. Referencenumeral 103 represents a red color filter. Blue light and green lightphotons have high energy and thus, are generally absorbed very quicklywithin a depth Xg defined from the surface of the silicon bulk to acertain region 104 thereof. On the other hand, red light photons havelow energy and penetrate a region deeper than the above region 104. Morespecifically, before generating any photoelectrons, the red lightphotons can penetrate to an interface 105 between an epitaxial substrateregion, located at a depth Xepi, and a highly doped P⁺-type substrate106. Reference letter ‘Xr’ denotes a depth of the interface 105 from thesurface of the silicon bulk (i.e., highly doped P⁺-type substrate 106).

When electrons 107 are generated in the highly doped P⁺-type substrate106, the electrons 107 recombine very quickly with the holes located inthe highly doped P⁺-type substrate 106 and cannot be collected in the“red” photodiode. Those electrons 108, on the other hand, which aregenerated in an un-depleted epitaxial layer 109, have much longerlifetime than the electrons 107, and diffuse freely in the un-depletedepitaxial layer 109 both laterally and vertically until the electrons108 reach the boundary of depletion regions 110. The boundary of thedepletion regions 110 is located at a depth Xd1 from the surface of thesilicon bulk.

When electrons 111 enter the depletion regions 110, the electrons 111are quickly swept into respective photodiode potential wells located inregions where N-type doped layers 112 are formed. The photodiodes areformed close to the surface of the silicon bulk by the N-type dopedlayers 112 and P⁺-type pinning layers 113. This structure is called thepinned photodiode. The P⁺-type pinning layers 113 each extend along thesides and the bottom of respective shallow trench isolation (STI)regions 114, each formed by etching the silicon bulk, to separate andisolate the photo sites and the corresponding electrical circuits fromeach other. The STI regions 114 are filled with silicon dioxide. Thesilicon dioxide also covers the photodiode surface area and extendsunder transfer gates 117. Reference numeral 115 and 116 respectivelyrepresent the silicon dioxide filling the STI regions 114 and thesilicon dioxide extending under the transfer gates 117 while coveringthe photodiode surface area. The transfer gates 117 are formed ofpolycrystalline silicon.

When a suitable bias is applied to each of the transfer gates 117 viacorresponding connections 118 (shown only schematically), electroncharge stored in the photodiode potential wells is transferred ontorespective FD nodes 119 formed by doping N⁺-type dopants. The FD nodes119 usually experience a voltage change. This voltage change is thensensed by suitable amplifiers (SFs), which are connected individually tothe FD nodes 119 by respective wires 120 (also shown onlyschematically). The voltage change represents a desired signal. Thephotodiodes and the transfer gates 117 are typically covered by anotherlayer 121, formed by silicon dioxide or multiple layers of silicondioxide, and other transparent films before color filters are depositedon the top. Microlenses (not shown in the drawing) are then alsodeposited on top of the blue, green and red color filters 101, 102 and103 to focus the light on the surface area of the photodiodes.

As can be easily understood from FIG. 1, those electrons generated bythe red light in the un-depleted epitaxial layer 109 can also diffuselaterally and enter the depletion regions 110 of the neighboringphotodiodes. This phenomenon often causes unwanted color crosstalk,since the red light-generated electrons usually end up in wrongphotodiode potential wells of the “green” or “blue” photodiodes. Thiscolor crosstalk may be pronounced in small size pixels where the lateraldimension of the pixel is less than 2 μm, while the vertical dimensionremains on the order of 5 μm. The color crosstalk can be reduced bydecreasing the thickness of the epitaxial layer (i.e., the depth Xr ofthe interface 105) and thus, reducing the thickness of the un-depletedepitaxial layer 109 or extending the boundary of the depletion regions110 located at the depth Xd1 to a depth Xd2.

However, the above-described two approaches may have some limitations.The shallow epitaxial thickness causes too many of the red lightelectrons to be generated in the highly doped P⁺-type substrate 106 andthus recombined with the holes in the highly doped P⁺-type substrate106. As a result, the red light electrons may not contribute to thesignal. It is usually desirable to have the epitaxial thickness on theorder of 5.0 μm or larger to have a good “red” light response.

The thick depletion that extends all the way to the interface 105 mayalso cause limitations. The low doping of the epitaxial layer that isnecessary to accomplish the thick depletion may increase the darkcurrent generation, and may lead to the discontinuity and separation ofthe P⁺-type pinning layers 113 located near the surface from the highlydoped P⁺-type substrate 106 as indicated by the separated depletionlayer boundaries 122 for this level of epitaxial doping. When thediscontinuous and separated P⁺-type pinning layers 113 are observed, itis necessary to provide other electrical connections to the P⁺-typepinning layers 113 by some other means such as metal wires placed overthe top of the pixels. These electric connections may reduce the pixelaperture efficiency and consequently the final pixel Quantum efficiency.

SUMMARY OF THE INVENTION

Specific embodiments of the present invention provide an image sensor(e.g. complementary metal-oxide semiconductor (CMOS) image sensor)including small size pixels and improved in color crosstalk.

In accordance with one aspect of the present invention, there isprovided an image sensor comprising a substrate of a first conductivitytype, first and second pixels arrayed over the substrate, and apotential barrier formed in a region of the substrate corresponding tothe first pixel but not in a region of the substrate corresponding tothe second pixel.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a simplified cross-sectional view of conventionalpixels overlaid with color filters in a CMOS image sensor.

FIG. 2 illustrates a simplified cross-sectional view of pixels overlaidwith color filters in a CMOS image sensor in accordance with anembodiment of the present invention.

DESCRIPTION OF SPECIFIC EMBODIMENTS

According to various embodiments of the present invention, small pixelsize sensors are improved in the performance, and the improvedperformance contributes to the reduction in color crosstalk. This effectcan be achieved by incorporating a deep high energy Boron implantationunder the pixels that receive the blue and green light but not under thepixels that receive the red light or by a low energy ion implantationapplied during the P-type epitaxial growth.

The implanted doping creates a small potential barrier in a substratestructure that directs and focuses those carriers generated by the redlight deep within the silicon bulk (i.e., the substrate structure) toflow into the “red” photodiodes (photodiodes under the red colorfilters) and are collected in the “red” photodiodes. The deep Boronimplantation also redirects carriers generated by the residual red lightpenetrating through the imperfect blue and green color filters andgenerating carriers deep within the silicon bulk under the “blue” and“green” photodiodes, so as to make the carriers generated under the“blue” and “green” photodiodes flow into the “red” photodiodes.

In addition to the reduction in the color crosstalk caused by the redlight-generated carriers, the color crosstalk caused by the imperfectblue and green color filters can also be reduced. As a result, it ispossible to build CMOS sensor arrays of pixels with very small size,high performance and reduced color crosstalk.

FIG. 2 illustrates a simplified cross-sectional view of an image sensorincluding pixels with photodiodes and corresponding transfer transistorsin accordance with an embodiment of the present invention. Blue, greenand red color filters 201, 202 and 203 are formed over an inter-leveltransparent dielectric structure 204, which is formed over photodioderegions. P⁺-type doped layers 217 form pinned photodiode regionstogether with N-type doped layers 210. The P⁺-type doped layers 217 eachextend over the exposed surface of a silicon bulk (e.g., a highly dopedP⁺-type substrate 218) including the sides and the bottom of shallowtrench isolation (STI) regions 206. The STI regions 206 are filled withan oxide-based material 207. Oxide-based layers 205 each cover thesurface of the silicon bulk and extend under the respective transfergates 208. The transfer gates 208 are formed of a conductive materialsuch as polysilicon. The N⁺-type doped layers 209 form FD regionsconnected to respective sense amplifiers (not shown).

When the photodiodes, more specifically, pinned photodiodes are depletedof all charge, depletion regions 211 are formed at a depth Xd1 under thepinned photodiodes. A high energy Boron implantation is used to form aP-type doped layer 212 as a potential barrier. The P-type doped layer212 is located at a depth Xg under “blue” and “green” photodiodes, wheremost of green and blue light photons have already been converted toelectrons 215. These electrons 215 drift upward for a short distance tothe boundary of the depletion regions 211 and are quickly swept intophotodiode potential wells located in the N-type doped layers 210. Sincethe vertical diffusion distance can be made very short, there is alittle chance of a lateral spread and thus a color crosstalk. Those redlight-generated electrons 216 also diffuse directly upward, since theP-type doped layer 212 forms a small potential barrier for the redlight-generated electrons 216 and prevents the lateral spread thereof.In addition, other red light-generated electrons 214 under the P-typedoped layer 212 cannot also overcome the potential barrier and need todiffuse around the potential barrier to the “red” photodiode potentialwells. As a result, the color crosstalk caused by the imperfect colorfilters can be improved.

For this reason, an epitaxial layer can have a suitable sufficient depthXr for an efficient conversion of the red light into electrons withoutthe need for a compromise to reduce the lateral spread into wrongphotodiodes. An epitaxial-substrate interface 213 can be placed evendeeper into the silicon bulk than the conventional epitaxial-substrateinterface to further improve the red light conversion to electrons.Reference letter ‘Xepi’ denotes a depth at which the epitaxial-substrateinterface 213 is located. The doping of the epitaxial layer can also beoptimized for a minimum dark current and a good conductive connection ofthe P⁺-type doped layers 217 to a P⁺-type doped substrate 218.Accordingly, a silicon bulk pixel aspect ratio, which is the effectivepixel silicon thickness to the pixel horizontal dimension, can beincreased without adverse effects on the color crosstalk in comparisonto the conventional approach.

As is well known, the wavelength of red light is the largest, anddescends in the order of green light and blue light. Therefore, the redlight-generated charge can be generated at a depth deeper than that ofthe P-type epitaxial layer. Hence, in consideration of this fact, theP-type doped layer 212 is formed at a suitable depth Xg from the surfaceof the silicon bulk.

Since the description above did not discuss the pixel circuits andfocused only on the photodiodes, it is understood that a shared pixelcircuitry may also be used in this embodiment of the present invention.Each pixel may have a shared circuit to read the photo-generated chargeas an electrical signal, and the shared circuit can read thephoto-generated charge through a shared floating diffusion node.

It is also clear to those skilled in the art that this embodiment of thepresent invention can be easily adapted to the 3T pixel structure, whichrepresents another embodiment of this invention. Furthermore, it isclear to those skilled in the art that the P-type doped layer 212 doesnot have to be implanted by a high-energy ion implantation. Instead, theP-type doped layer 212 may be formed during the epitaxial layer growth.The epitaxial growth can be stopped at the depth defined between thedepth Xr and the depth Xg. Boron may be implanted by a low energy ionimplantation or deposited by some other means. Afterwards, the epitaxialgrowth can continue until reaching the original depth Xr. This approachrepresents another embodiment of the invention.

Additionally, it is clear to those skilled in the art that the P-typedoped layer 212 can be formed at the depth Xg under the “green”photodiode, and another similar P-type doped layer can be formed at ashallower depth Xb (not shown) than the depth Xg under the “blue”photodiode. This approach represents another embodiment of the presentinvention.

In the present embodiment, the color crosstalk originating from thecarriers generated in the deep region of the substrate structure (e.g.,silicon bulk) can be minimized by placing the P-type doped layer, whichfunctions as a potential barrier, under the “green” and “blue”photodiodes and not under the “red” photodiode. Accordingly, it ispossible to provide a solid-state image sensor, more particularly, CMOSimage sensor that has a small pixel size, good response to the redlight, and less occurrence of color crosstalk.

Various embodiments of the present invention are directed toward thepixels that have an improved crosstalk for the small pixel size, whichwas accomplished by incorporating deep P-type layers under the “green”and “blue” photodiodes and not under the “red” photodiodes. However,this improvement is intended to be illustrative and not limiting, and itshould be noted that the persons skilled in the art can makemodifications and variations in light of the above teachings. It istherefore to be understood that changes may be made in the particularembodiments of the invention disclosed, which are within the scope andspirit of the invention as defined by appended claims.

1. An image sensor comprising: a first pixel including a firstphotodiode; a second pixel including a second photodiode; a silicon bulkbeneath the first photodiode and the second photodiode; a potentialbarrier beneath the first photodiode; and an epitaxial layer including:a first region between the potential barrier and the first photodiode;and a second region between the potential barrier and the silicon bulk;wherein the first region is configured to receive a photon of a firstcolor, responsively generate a first electron, and sweep the firstelectron into the first photodiode; wherein the second region isconfigured to receive a photon of a second color, responsively generatea second electron, and sweep the second electron into the secondphotodiode; and wherein the second region and the potential barrier areconfigured to redirect the second electron around the potential barrierand into the second photodiode.
 2. The image sensor of claim 1, whereinthe photon of a second color comprises a red photon.
 3. The image sensorof claim 1, wherein the photon of a first color comprises at least oneof a blue photon or a green photon.
 4. The image sensor of claim 1,wherein the silicon bulk comprises a highly-doped P⁺-type substrate. 5.The image sensor of claim 1, wherein the first and second photodiodescomprise a pinned photodiode.
 6. The image sensor of claim 1, whereinthe potential barrier comprises a P-type doped layer.
 7. The imagesensor of claim 1, further comprising: a third pixel including a thirdphotodiode; wherein the silicon bulk is located beneath the thirdphotodiode; wherein the potential barrier is located beneath the thirdphotodiode; wherein the first region is located between the potentialbarrier and the third photodiode; and wherein the first region isconfigured to receive a photon of a third color, responsively generate athird electron, and sweep the third electron into the third photodiode.8. The image sensor of claim 7, wherein a depth between the firstphotodiode and the potential barrier is greater than a depth between thethird photodiode and the potential barrier.
 9. An image sensorcomprising: a first photodiode; a second photodiode; a potential barrierincluding a potential barrier thickness and a potential barrier top; anda silicon bulk; wherein the top of the potential barrier is located at apotential barrier depth below the first photodiode; wherein the siliconbulk is located at a silicon bulk depth below the first photodiode; andwherein the silicon bulk depth is greater than the sum of the potentialbarrier depth below the first photodiode and the potential barrierthickness.
 10. The image sensor of claim 9, further comprising: a thirdphotodiode; wherein the top of the potential barrier is located at apotential barrier depth below the third photodiode; and wherein thepotential barrier depth below the first photodiode is greater than thepotential barrier depth below the third photodiode.
 11. The image sensorof claim 9, wherein the potential barrier is not located substantiallybeneath the second photodiode.
 12. The image sensor of claim 9, whereinthe first and second photodiodes comprise a pinned photodiode.
 13. Theimage sensor of claim 9, wherein the potential barrier comprises aP-type doped layer.
 14. The image sensor of claim 9, wherein the firstphotodiode is located within at least one of a green pixel or a bluepixel.
 15. The image sensor of claim 9, wherein the second photodiode islocated within a red pixel.